Storage device counting error correction

ABSTRACT

In the conventional nonvolatile memory, it is not possible to determine the cause of the error is accidental or due to the degradation when the error is detect at the time of data read. Therefore, unnecessary substitute processing is performed, resulting in the exhaustion of the substitute area to shorten the life of the storage device.  
     The semiconductor storage device according to the present invention determines the cause of an error at the time of the error correction of data read out from a nonvolatile semiconductor memory, on the basis of a previously recorded error correction count, and selects a data refresh processing or a substitute processing to perform. When the error is detected, the corrected data is rewritten back for preventing reoccurrence of error due to accidental cause. While, when it is determined that the reoccurrence frequency of the error is high and the error is due to degradation of the storage medium, based on the error correction count, the substitute processing is performed.

[0001] This application claims a priority based on Japanese patentApplication 11-263156 filed on Sep. 17, 1999, the entire contents ofwhich are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a storage device having asemiconductor memory as a storage medium, particularly to a storagedevice using an error correction code for improving reliability ofstored data.

[0003] Rapid increase in kinds and amounts of information data with thedevelopment of information processing equipment has brought a remarkabledevelopment of storing devices. For accelerating expansion of storagecapacity, efforts have been directed to technological developments forincreasing storage densities of storage media. Nevertheless, when datais stored in a temporary storage medium, namely in a rewritable storagemedium, the security of the stored data is not always assured becausephase change of magnetic material or storage of electrical charges isutilized. The reliability of stored data is generally lowered as thestorage density is increased.

[0004] In a storage device with a flash memory, for example, electricalcharges stored in a storage element sometimes gradually leaks out tocause destruction of data. Therefore, almost all storage devices of sucha kind are provided with data protection functions using errorcorrection codes. In an external storage device used for a personalcomputer, an error correction code, which is similar to the one used ina magnetic disk drive and calculated using 512 byte data, are storedtogether with data. When read out, the stored data are checked forerrors, the errors are corrected if errors are detected, and then storeddata are outputted with the security thereof being increased.

[0005] In the above conventional magnetic storage device, no problem iscaused by stochastically occurred errors, namely, errors that do notreoccur in the same location. However, in the storage device, noconsideration is given for errors fixed to a certain location due toaccidents inherent to a storage medium such as fatigue, damage, foreignmatter sticking, and wire breaking in the storage medium. If such kindsof errors occur with negligibly small probabilities, there is noproblem. However, such kinds of errors will accumulate when they occurwith probabilities that are not negligible in the normal use. The errorcorrection code is usually capable of correcting errors with a specifiednumber of bits or in a specified area. However, if there occurs errorsexceeding error correction capability of the code, the error can not bedetected (miss detection), or wrong identification of an error locationor an error pattern can be made to perform faulty correction of correctdata (miss correction). Therefore, even if data is corrected when beingoutputted, the incorrect data left uncorrected on the storage mediumcauses another error that accumulates to exceed the capability of theerror correction code, which results in making it impossible to recoverthe stored data. Only with a measure for increasing reliability bymerely using error correction code, the foregoing problem will occur.

[0006] In addition, there can be another problem due to occurrences oftwo kinds of errors characteristic to the storage medium: an occurrenceof data error due to degradation of the medium and that due toaccidental causes. In a nonvolatile semiconductor memory, for example,degradation due to repetition of a rewriting cycle deteriorates datamaintain characteristics to cause data error. Meanwhile, electric signalnoises or radiation can cause stochastic data errors irrespective of thedegradation. In this case, by rereading the data, correct data can besometimes read, or by rewriting the data, correct data can be readthereafter. However, these two kinds of data errors can not bedistinguished from each other by observing the data which are theresults of the errors. In the case of the data error due to degradation,since it will accumulate in a area as described above, a substitute areamust be provided for storing data and the degraded area must be madeunusable without being left as it is to avoid accumulation of errors. Inthe case of the data error which is accidental and does not relate todegradation, since an area with such error can be brought normal stateby rewriting the data, providing a substitute area and making the errorarea unusable is to lower the efficiency of the storage device. However,no measure has been taken for such a problem as above.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to increase security ofthe data in a nonvolatile semiconductor storage device and to extendlife of the nonvolatile semiconductor storage device by deciding whetherthe occurred data error is of accident or of reoccurrence due todegradation and carrying out the suitable processing.

[0008] In a nonvolatile semiconductor storage device according to thepresent invention, first, when a data error is detected in reproducingdata from each block of a storage medium, which block is a unit of dataaccess, error correction is carried out with the error correction countrecorded for each area. Second, it is judged for each block whether theerror correction count reaches a predetermined value or not. If theerror correction count is less than the predetermined value, correcteddata is rewritten in the same area. If the error correction count hasreached the predetermined value, a substitute area is allocated to whichthe corrected data is transferred with the previously used area madeunusable.

[0009] Moreover, with the degree of error correction divided into aplurality of grades, rewriting the data or allotting a substitute areais selected to be executed in accordance with the grade.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic diagram showing a configuration of a storagedevice according to the present invention;

[0011]FIG. 2 is a schematic diagram showing an example of a managementinformation;

[0012]FIG. 3 is a flow chart showing a reading processing;

[0013]FIG. 4 is an example of a table for deciding whether a refreshingprocess is to be carried out or a substitute process is to be carriedout;

[0014]FIG. 5 is a schematic diagram showing a data flow in a refreshprocessing;

[0015]FIG. 6 is a flow chart showing an operation of the refreshprocessing;

[0016]FIG. 7 is a schematic diagram showing a data flow in a substituteprocessing; and

[0017]FIG. 8 is a flow chart showing the substitute processing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Embodiments of the present invention will be explained in thefollowing with reference to drawings.

[0019]FIG. 1 is a schematic diagram showing a configuration of asemiconductor storage device, in particular, of a nonvolatilesemiconductor storage device. The storage device 101 according to thepresent invention shown in FIG. 1 is connected to a system bus 102 of ahost computer that makes a request to the storage device 101 for datareading and writing. A rewritable nonvolatile memory 103, which is astorage medium of the storage device 101 and can be in various formssuch as a chip, module, and unit, includes a normal data storing area104, a normal management area 105, a substitute data storing area 106,and a substitute management area 107. The normal data storing area 104,which is a normally used area, is divided into a plurality of blocks,each of which is a unit of data access. The divided block units of thenormal data storing area 104 are managed by the normal management area105 that stores management information of the normal data storing area.The substitute data storing area 106 is divided into a plurality ofblocks, each of which is allocated for being substituted for a faultyblock in the normal data storing area 104. The divided block units inthe substituted data storing area 106 are managed by the substitutemanagement area 107, in which there is stored management information forthe substitute data storing area 106 (correspondence to the normal datastoring area 104, error detection and correction codes of thesubstituted data, etc). That is, in the data storage device according tothe present invention, the data storing areas are controlled in blocks,each of which is a unit of data access. The storage unit 101 furtherincludes a control unit 108 for carrying out control inside the storageunit 101, an error detection and correction circuit 109 for the datastored in the nonvolatile memory 103, and a buffer memory 110 thattemporarily stores data received from and transmitted to the system bus102.

[0020] Next, an operation of the storage device 101 will be explained.The host computer requests the storage device 101 through the system bus102 to store data or to read out stored data. The control unit 108identifies a physical address in the nonvolatile memory 103corresponding to a logical address indicated at the same time when therequest is made by the host computer, and carries out a processing inaccordance with a requested access.

[0021] Since the data storage device 101 according to the presentinvention excellently exhibits its effect at reading of the stored data,the read operation will be explained. When reading of a certain data isrequested, the control unit 108 identifies a corresponding physicaladdress of the requested data in the nonvolatile memory 103 and read outthe data in the area corresponding to the physical address in the normaldata storing area 104 and the management information in thecorresponding area in the normal management area 105. Both of the dataand management information being read out are inputted to the errordetection and correction circuit 109 to be subjected to error detection,and are also stored into the buffer memory 110.

[0022]FIG. 2 is a schematic diagram showing an example of contents ofmanagement information in the normal management area 105. A data areaerror detection and correction code (ECC) 201 for detecting andcorrecting an error or errors in a data area is obtained by inputtingstored data in the error detection and correction circuit 109. If theresult of the error detection indicates that error correction isrequired and possible, the error correction count 202 is recorded foreach divided block in the corresponding data storing area. A blockidentifier code 203 indicates a content of a block. With the blockidentifier code 203, there are identified several kinds of blocks suchas a usable block, unusable block, normal data block, substitute areablock, and empty block. In an area 204, a logical address 204 is storedwhich is assigned by the host computer when the block is allocated. Inan area 205, there is stored a management area error detection andcorrection code 205 for increasing the reliability of the managementinformation itself.

[0023] The management information shown in FIG. 2 is stored in eachblock in the normal management area 105 and the substitute managementarea 107. All of the information are produced in the control unit 108and stored into the corresponding locations in the normal managementarea 105 or the substitute management area 107. The control unit 108, onthe basis of the information, identifies a physical storing location(physical address) of a read data requested by the host computer andreads out a corresponding location in the normal data storing area 104for responding to the access request of the host computer.

[0024] When an error is detected in read out data in the read operation,a processing is carried out as explained below. The control unit 108inputs the data read out from a specified block in the nonvolatilememory 103 into the error detection and correction circuit 109 togetherwith the data area error detection and correction code 201 in themanagement information to detect presence or absence of an error in thedata. When the presence of an error is detected, it is meant that thereoccurs an error of some kind in the nonvolatile memory 103. Although theerror may be caused by a transmission error on some transmission line inthe device, here, the error is taken as being caused by a failure ofsome kind in the nonvolatile memory 103. Data with an error should bereused if the error is due to an accidental cause and the data can beused without problem by rewriting it with the error being corrected.While, however, an error caused by degradation is anticipated to reoccureven if the data is rewritten with the error being corrected. Therefore,in the embodiment, after correcting the error, the data is rewritten forthe present with an error correction count recorded to determine thecause of the error based on the error correction count.

[0025] An operation procedure in a read processing is shown in a flowchart in FIG. 3. The flow chart is shown to be started from a step atwhich a read access is requested from the host computer through thesystem bus 102. The control unit 108 specifies a physical storagelocation to read data and management information (STEP 301). At thistime, the read out data and the management information are inputted tothe error detection and correction circuit 109, by which it can beascertained whether an error has occurred or not (STEP 302). When noerror has occurred in the data at this time, the data can be provided tothe host computer as is read out, therefore the read out data isoutputted (STEP 310). While, when an error is detected, a decision ismade as to whether the error is correctable or not on the basis of theprocessing result of the error detection and correction circuit 109(STEP 303). When there has occurred an uncorrectable error at this time,it is of no use to read out the data. Thus, it is informed to the hostcomputer that the data can not be read out (STEP 304). When there hasoccurred the correctable error, the error correction isperformed(STEP305) with an increment of the error correction count toupdate the management information (STEP 306).

[0026] Following this, the error correction count is compared with theerror correct maximum value(ECM) (STEP 307). When the error correctioncount is less than ECM, the data area is determined to be still usable,and the corrected data is rewritten at the same location and the dataarea is continuously used (refresh processing, STEP 308). While, whenthe error correction count becomes equal to or more than ECM, it isdetermined that the data area is not usable any more and substituteprocessing is carried out (STEP 309). Namely, an unused block isselected from the substitute data storing area 106 to be allocated as asubstitute area. At this time, it is necessary to prepare informationindicating the correspondence of the substitute area and the normal datastoring area 104 and to write the information in the correspondinglocation in the management information. On completion of the processingat STEP 308 or STEP 309, the corrected data is outputted to the hostcomputer to complete the read processing (STEP 310).

[0027] In the above embodiment, switching between the rewritingprocessing and the substitute processing is carried out in accordancewith the decision as to whether the error correction count exceeds thepredetermined count (ECM) or not. In addition, as shown in FIG. 4,together with the error correction count, the error occurrence count inthe data block can be taken as an additional factor in making suchdecision. There occurs not always one error in each data block, but aplurality of errors can occur at a time. A high count of erroroccurrence indicates that the block has a high possibility of beingdegraded or falling into a state in which the data correction isimpossible. Therefore, such a block may be better subjected to thesubstitute processing in an earlier stage. The table in FIG. 4 indicatesthat when many errors occur, the substitute processing is carried outdespite small error correction count. In this case, an error occurrencecount is also recorded in the error correction count 202 shown in FIG. 2together with the correction count. Along with this, at STEP 307 in FIG.3, a decision is made on the basis of the table shown in FIG. 4 as towhether the refresh processing or the substitute processing is to becarried out.

[0028]FIG. 5 is a schematic diagram illustrating a data flow in a datarewriting operation in the refresh processing, and FIG. 6 is a flowchart showing the data rewriting operation in the refresh processing. Anarea 501 is a stored data section of a certain block in the normal dataarea. An area 502 is for an error detection and correction code (ECC) ofthe stored data. An area 503 is for miscellaneous managementinformation, an example of which is shown in FIG. 2. An area 504 is abuffer memory for temporarily storing the read out stored data. The readout stored data 501 is inputted to the error detection and correctioncircuit 109 and at the same time, stored in the buffer memory 504 (STEP601).

[0029] At this time, the ECC 502 is also inputted to the error detectionand correction circuit 109 to carry out the error detection. When anyerror is detected, decision is made, on the basis of the number oferrors and/or the error correction count as shown in FIG. 4, as towhether the data rewrite processing (refresh processing) is to beexecuted or not (STEP 602). When it is decided that no refreshprocessing is to be executed, other processing is carried out tocomplete the processing (STEP 603). While, when it is decided that therefresh processing is to be executed, the error detection and correctioncircuit 109 is further operated to identify the error location and theerror pattern to execute error correction on the buffer memory 110 (STEP604). Next to this, an increment of the error correction count iscarried out (STEP 605) with the stored data written back in the samelocation (STEP 606). The foregoing operation is different from that inthe previously explained processing in the order of carrying out theprocessing. However, since there is no problem in obtaining a requiredeffect, the order can be changed as it fits.

[0030]FIG. 7 is a schematic diagram illustrating a data flow in thesubstitute processing in which an operation of data correction is thesame as that in the refresh processing illustrated in FIG. 5 withfurther operation carried out thereafter for storing the corrected datain another selected block 701. At this time, since the stored data isnot modified, the ECC 502 is to be the same. Therefore, the ECC 502 iswritten as it is as an ECC 702 when there occurs no error therein.While, when there occurs any error in the ECC 502, it is to be writtenas the ECC 702 with the error corrected. In addition, miscellaneousinformation 503 (management information other than ECC) is to be writtenwith necessary updating.

[0031] An entire flow of the substitute processing is shown as a flowchart in FIG. 8. The substitute processing shown in FIG. 8 is executedat STEP 309 in the flow chart shown in FIG. 3. At first, at STEP 801,there is carried out a selection of a substitute block for the correcteddata. Of various possible ways of selecting the substitute block, thatof using blocks in order in an area specialized for substitutionprovided beforehand (for example, the substitute data storing area 106shown in FIG. 1) may be most easily managed with a short selection time.Other possible ways are such ones as selecting a block with smallerdegradation, and using a block predetermined so as to correspond to theblock to be substituted.

[0032] Following this, at STEP 802, updating of a management table iscarried out which becomes necessary by a change in the correspondencebetween the logical address and physical address of the block due to thesubstitution. The management table is the one such as an addresstranslation table for translating a logical address (an addressspecified by the host computer in accessing) into a physical address ona memory. It is indispensable to a storage device that carries outsubstitution. The management table can be managed in various ways. Thepresent embodiments store the whole management table in a specifiedlocation (for example, the substitute management area 107 in FIG. 1) onthe nonvolatile memory for facilitating management and updating. Oncompletion of the updating, the data is transferred to the substituteblock and stored in the block at STEP 803. Since the corrected data isstored in the buffer memory, the stored corrected data is thentransferred to the substitute data storing area. The ECC data and themanagement information are processed as previously described. Thus, thesubstitute processing is completed.

[0033] According to the present invention, it is decided whether thesubstitute processing is to be executed or not by determining whether anerror occurrence is due to accident or due to degradation that causesfrequent occurrence of the error. Therefore, the substitute processingcan be executed less frequently to make it possible to reduceunnecessary use of the substitute area. Moreover, with the rewriting ofthe data, the cell status can be made more stable in the one where dataerror occurred as well as the one where the error is not occurred yet,so that the data can be stored with increased reliability. Furthermore,in addition to the error correction count, the error occurrence countalso can be taken as an additional factor of the above decision to allowa practical and fine control.

[0034] While the present invention has been particularly shown anddescribed with reference to preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details can be made therein without departing fromthe spirit and scope of the present invention.

What is claimed is:
 1. A storage device comprising: a semiconductormemory for storing data, the memory including a plurality of blocks;means for reading the data from each block of the memory; errordetection and correction means for detecting and correcting an error ofthe read data for each block; and means for recording an errorcorrection count about the read data for each block.
 2. A storage deviceas claimed in claim 1 wherein a block, in which a predetermined count oferror correction was carried out, is decided to be unusable on the basisof a result of the record of an error correction count.
 3. A storagedevice as claimed in claim 2 wherein a substitute block is allocated tothe block which is decided to be unusable.
 4. A storage devicecomprising: a semiconductor memory for storing data, the memoryincluding a plurality of blocks; means for reading the data from eachblock of the memory; error detection and correction means for detectingand correcting an error of the read data for each block; and means forrewriting data in an area in which error correction is carried out, backin the same read area when an error is detected in the read data.
 5. Astorage device comprising: a semiconductor memory for storing data, thememory including a plurality of blocks; means for reading the data fromeach block of the memory; error detection and correction means fordetecting and correcting an error of the read data for each block; andmeans for rewriting data in an area in which error correction is carriedout in another area when an error is detected in the read data.
 6. Astorage device comprising: a semiconductor memory for storing data, thememory including a plurality of blocks; means for reading the data fromeach block of the memory; error detection and correction means fordetecting and correcting an error of the read data for each block; meansfor recording an error detection count about the read data for eachblock; and means for carrying out a comparison of a value of the errordetection count with a predetermined value when an error is detected inthe read data, and for rewriting data in an area in which errorcorrection of the data is carried out in one of the same area andanother area on the basis of a result of the comparison.
 7. A storagedevice comprising: a semiconductor memory for storing data, the memoryincluding a plurality of blocks; means for reading the data from eachblock of the memory; error detection and correction means for detectingand correcting an error of the read data for each block; means forrecording an error detection count and an error occurrence count aboutthe read data for each block; and means for carrying out comparisons ofvalues of the error detection count and the error occurrence count withrespective predetermined values when an error is detected in the readdata, and for rewriting data in an area in which error correction of thedata is carried out, in one of the same area and another area on thebasis of results of the comparisons.
 8. A storage device comprising: asemiconductor memory for storing data, the memory including a pluralityof blocks; means for reading the data from each block of the memory;error detection and correction means for detecting and correcting anerror of the read data for each block; means for recording an erroroccurrence count about the read data for each block; and means forcarrying out a comparisons of a value of the error occurrence count witha predetermined values when an error is detected in the read data, andfor rewriting data in an area in which error correction is carried out,in one of the same area and another area on the basis of a result of thecomparison.
 9. A storage device comprising: a nonvolatile semiconductormemory for storing data, the memory including a plurality of blocks;means for reading the data from each block of the memory; errordetection and correction means for detecting and correcting an error ofthe read data for each block; and means for recording an error detectioncount about the read data for each block, said nonvolatile semiconductormemory including a normal data storing area, a normal management areastoring management information including the error correction count of anormal data, a substitute data storing area which is a substitute for afaulty data storage area, and a substitute management area for managingthe substitute data storing area.